1. Technical Field
Aspects of the invention relate generally to light emitting devices, and more specifically, to a shaped contact layer for a light emitting device.
2. Background Art
Recently, a great deal of interest is focused on nitride-based light emitting diodes (LEDs) and lasers that emit light in the blue and deep ultraviolet (UV) wavelengths. These light emitting devices may be capable of being incorporated into various applications, including solid-state lighting, biochemical detection, high-density data storage, and the like. However, to date, the performance of nitride-based light emitting diodes and lasers quickly worsens as the radiation wavelength is reduced into the ultraviolet range.
For example, an Aluminum (Al)-Gallium (Ga)-Nitrogen (N)-based UV LED includes an n-type AlGaN contact layer for which the current flow is lateral. For short wavelength light emitting devices, a molar fraction of Al in the AlGaN contact layer is increased to provide a low absorption of the UV light emitted by the device. However, contact layers having a high Al molar fraction tend to have a much higher resistivity, thereby resulting in a high resistance for the contact layer and current crowding. As a result, the device will tend to have a non-uniform distribution of light.
A characteristic current crowding length, Lcr, for a UV LED can be estimated as
            L      cr        =                  qIR        sq                    η        ⁢                                  ⁢                  k          B                ⁢        TW              ,where q is the electron charge, I is the device current in Amperes; Rsq is the resistance of the lateral contact layer in ohms per square (Ohm*m2); f is the diode ideality factor; kB is the Boltzmann constant, kB=1.38×10−23 J/K; T is the diode operating (junction) temperature in degrees Kelvin; and W is the contact periphery in meters. An estimate for Lcr can be obtained based on an assumed uniform current distribution along a perimeter of the LED.
An approach seeks to decrease the effect of current crowding in an LED by manufacturing the LED so that a distance from the metal contact to the center of the LED is smaller than or comparable to Lcr. Using this approach, a long “stripe” LED is the most efficient shape. However, such an LED does not provide a practical beam shape and is not conducive to LED packaging solutions. A more conventional circular shape for the LED has the smallest active light emitting area for a given device perimeter, W. Additionally, the circular shape is not optimal for flip chip packaging of the LED.
In view of the foregoing, a need exists to overcome one or more of the deficiencies in the related art.